Today's first word is
勿体無い もったいない mottainai
It means 'it's a waste', 'wasteful', 'worth keeping'. For instance, my grandmother used to keep little bits of string because she considered it to be もったいない to throw them away. Letting the tap run when you're brushing your teeth, throwing away plastic bags and so on were all もったいない for her. (Aren't most grandmothers like this? They are from a generation where you needed to carefully conserve your resources and possessions.)
The word もったいない has been appropriated by the green/eco movement in recent years. There is even a non-profit organization called MOTTAINAI.
A big もったいない situation for Japanese people, especially older folk, is to leave grains of rice in your rice bowl. Rice has a special place in Japanese culture, and wasting it is several levels worse than wasting any other kind of food.
Besides もったいない、the other word that comes into play is:
申し訳ない もうしわけない moushiwakenai
This is a bit difficult to translate to English, but it means feeling ashamed, being apologetic. Used on its own to address someone, it means "I'm sorry". (You saw a lot of Japanese bankers using this phrase in the early '90s or so, while bowing their heads deep to the ground.)
So, whenever I'd leave little bits of rice in my rice bowl growing up, one of my parents would invariobly say もったいない, and then お百姓様に申し訳ない.
ohyakushousama ni moushiwakenai
お百姓様 is a very polite word for farmer(s). So the entire phrase means, to feel shameful or apologetic towards the honorable farmer(s) who grew the rice.
Think of the starving children in (blank)
Another thing my parents used to say when we left food uneaten was, 'Think of the children in (some starving country)'.
(country) の子供のことを考えなさい (country) のこどものことをかんがえなさい - (country) no kodomo no koto o kangaenasai
I thought it was only Japanese parents who did this, but I've since found out that parents all over the place say the same thing! I always thought that it was illogical. Why would me clearing my plate when I was full already, help the poor starving children in (some starving country)? But then, who said parents had to be logical...
You could probably guess the age of someone by which starving country their parents used to guilt them into cleaning their plates. For me, it was usually Ethiopia and Bangladesh I think. For an older generation in America it was the starving children of Europe. What country was it for you and your parents? Do you use the same tactic for your kids?